The present invention relates to a heart rate acceleration/deceleration detection system. The system has particular utility as part of an automatic implantable defibrillator (or cardioverter) that provides high-energy electrical pulses directly to the heart in response to the detection of a life-threatening arrhythmia.
In recent years, substantial progress has been made in the development of techniques for effectively defibrillating or cardioverting various heart disorders and arrhythmias. Past efforts have resulted in the development of implantable electronic standy defibrillators which, in response to the detection of an abnormal cardiac rhythm, discharge sufficient energy via electrodes connected to the heart to depolarize and restore it to normal cardiac rhythm.
Research efforts have also been directed toward developing techniques for reliably monitoring heart activity in order to determine whether defibrillation of cardioversion is necessary. Such techniques include monitoring the heart rate or determining the presence of fibrillation on the basis of a probability density function (PDF). A system using the PDF technique statistically evaluates the time the cardiac waveform spends away from the zero-potential axis. When the waveform becomes dangerously irregular, as measured by its probability density function, an abnormal cardiac function requiring defibrillation or cardioversion is suggested. This PDF technique is described in commonly owned U.S. Pat. Nos. 4,184,493 and 4,202,340, both of Langer et al.
Another system for monitoring heart activity utilizes both the PDF technique to determine the presence of an abnormal cardiac rhythm and a heart rate sensing circuit for indicating, with the PDF technique, ventricular fibrillation and high rate tachycardia (indicated by heart rate above a predetermined minimum threshold). Upon the detection of high rate tachycardia, i.e., upon the heart rate sensing circuit output exceeding a predetermined threshold, an arrhythmia condition is assumed and a defibrillating or cardioverting pulse is issued. A typical heart rate detection circuit is shown in commonly owned U.S. Pat. No. 4,393,877 to Imran et al. Another heart rate detecting circuit using an automatic gain control (AGC) feedback circuit is described in co-pending U.S. patent application Ser. No. 478,038 filed on Mar. 23, 1983 (a continuation of Ser. No. 370,191, filed on Apr. 21, 1982) to Imran et al, now U.S. Pat. No. 4,614,192.
As discussed in U.S. Pat. No. 4,393,877, a defibrillating pulse may be issued in response to outputs from both the PDF circuit and the heart rate averaging circuit or in response to the heart rate averaging circuit alone. That is, in certain circumstances it may be desirable to treat an arrhythmia solely in response to the average heart rate exceeding a predetermined threshold.
With some patients, however, the mere presence of an average heart rate above a predetermined threshold may not warrant the delivery of a defibrillating or cardioverting pulse. For example, certain patients, despite requiring a defibrillator implant, may still be able to engage in relatively strenuous exercise which could cause their heart rate to exceed the threshold level that would normally indicate an arrhythmia. For such patients, it is important to protect against the implanted device issuing an unnecessary and unwanted high energy pulse in response to such high heart rate.
It is thus seen that a need exists for an implantable defibrillator or cardioverter that is able to distinguish between high rate tachycardia that requires a corrective defibrillating or cardioverting shock and high rate normal sinus rhythm resulting from strenuous exercise or the like that does not warrant treatment.